1. Field of the Invention
The present invention relates to a so-called field sequential format image display device.
2. Related Background Art
(1) Conventionally, image display devices for displaying all sorts of information are being used in various fields, as is explained below.
In recent years, there have been more and more situations in which large-screen display devices are used for enjoying movie, television images and home video images in the home, and for using various picture image sources, such as for presentations and television conferences, in the office.
Furthermore, in contrast to the case of sequentially scanning multiple display value picture elements inside the screen to display an image as in the case of conventional CRT and liquid crystal, there exists a display device using a binary display picture element and performing time divisional display based on pulse width modulation (PWM) per each indicated value in order to realize a multiple gradation display. Examples of display devices for performing such time division display include plasma display; ferroelectric liquid crystal (FLC); rear-type projection television using binary-displayable spatial modulation elements (image display elements) being represented by MEMS (micro-electro-mechancial systems)-type elements such as those of the DMD devices of Texas Instruments (TI); and projection-type projectors.
Furthermore, it is desirable to realize a display portion of which the construction itself is concise, and thus inexpensive and light, so as to provide a product that may be easily purchased by a consumer.
(2) In recent years, colorization of the display is desired. One method for this is a so-called field sequential method. Hereinafter, explanation is made of this method.
In the case of a projection-type display device such as a projector, one layer of the spatial modulation element is used to display the image in each color sequentially, and synthesis thereof is performed by the vision faculties of the viewer. Thus, for realizing a single display, the single-plate sequential color-switching-format display device not only requires only one third of the cost of the conventional three-plate format for the spatial modulation elements and peripheral circuitry, but also, the optical system and the electrical circuitry system, for example, are simplified. This is one method for realizing an inexpensive and light display portion.
Methods of sequentially switching the color include a format such that a color filter is made out of a high-speed responding liquid crystal and switching is performed therewith, and a format such that a disc-shaped color filter is made to rotate and switching is performed therewith, for example.
(3) Explanation is made of an image display device using this field sequential format, making reference to an example of a conventional construction in FIG. 8.
A conventional image display device 200, as shown in FIG. 8, is equipped with an image display element 2 for modulating incident light and displaying a black-and-white image, and is constructed so that light is irradiated to a display element 2 from an illumination device 203. This illumination device 203 is constructed of a metal halide lamp 30 for irradiating white light; one disc-shaped color filter plate 231 arranged between the lamp 231 and an image display element 2; and a filter rotating unit 233 for rotating the color filter plate 231. Further, by rotating the color filter plate 231 at a fixed speed by means of the filter rotating unit 233, a construction is realized such that light of each color is sequentially radiated to the display element 2. Here, the term “black-and-white image” is used in contrast to “color image”, and does not refer to binary gradation. For example, the image display element 2 of the present embodiment is a binary display device, but multiple-gradation display may be realized by performing time division operations.
The illumination device 203 is constructed as described above. Thus, the images displayed to the image display element 2 are sequentially switched in synchronization with the sequential radiation of each color of light. This produces the result that the image is displayed to the screen 4 in each color. The color images in multiple colors are visually mixed, and thus recognized as a full color image.
Note that, the color filter plate 231, as shown in detail in FIG. 9, is divided into the color areas 231R, 231G and 231B, red, green and blue, respectively. Reference numeral 231a indicates the rotational center of the color filter plate 231.
(4) Hereinafter, explanation is made of a detailed construction of the image display device, for reference.
Reference numeral 7 is an image signal input unit. Reference numeral 8 is a signal processing unit for adjusting image qualities of an inputted picture image signal such as brightness, color characteristics and gamma characteristics, performing image signal processing of converting the inputted picture image to a time division signal based on a pulse width modulation suitable for driving the display element, and generating, for example, a pulse for driving the display element and a motor control signal. Reference numeral 8a is a data bus for transmitting the time division signal to the display element, and reference numeral 8b is a control line for transmitting the drive pulse to the display element.
Reference numeral 2 is the ferroelectric liquid crystal (FLC) or the binary display image display element represented by the MEMS (micro-electro-mechanical systems)-type element such as the DVD device from Texas Instruments (TI), and is a reelection-type display element for reflecting light.
Reference numeral 35 is a ballast power source for lighting up the metal halide lamp 30. Reference numeral 5 is an optical system for projection for projecting reflected light from the image display element 2 to a screen 4. Lenses 37 and 38 are arranged between the color filter member 231 and the lamp 30, and between the color filter member 231 and the image display element 2, respectively.
Reference numeral 360 is a detection sensor for detecting the rotational position of the color filter member 231. By means of this detection sensor 360, the rotational phase of the color filter member 360 is communicated to a unit of controlling filter motor within a signal processing unit 8. In order to achieve synchronization of the signal for driving the display element with the rotation phase of the color filter, the unit of controlling filter motor controls the motor control signal by means of a PLL circuit or the like and drives the rotating filter driving portion 233 via a signal line 62.
Next, explanation is made with reference to FIG. 10 of a construction of the above-mentioned signal processing unit 8. Here, FIG. 10 is a block diagram showing a detailed construction of the signal processing unit 8.
The image signal input unit 7 has: an image signal input terminal 71; an input terminal 72 for input of a horizontal synchronization signal for the input signal (IHD); an input terminal 73 for input of a vertical synchronization signal for the input signal (IVD); and an input terminal 74 for input of a clock of the input signal (ICLK).
In the diagram, reference numerals 711, 712, 713 and 714 indicate a data bus for the image signal. Reference numeral 721 indicates a signal line of the horizontal synchronization signal for the input signal (IHD), reference numeral 732 indicates a signal line of the vertical synchronization signal for the input signal (IVD), and reference numeral 741 indicates a signal line of the input signal clock (ICLK).
Reference numeral 80 is an image input unit being a receiving unit for receiving an image signal, such as a decorder for decoding, upon receiving a signal in TMDS format that is an image transmission format adopted in DVI (Digital Visual Interface) standard standardized by a standardization organization DDWG (Digital Display Working Group), the signal into 24-bit data (8 bits per RGB) or a decorder for decoding, upon receiving a compressed signal in MPEG format transmitted via IEEE 1394, the signal into 24-bit data (8 bits per RGB).
Reference numeral 81 is a format converter being a portion for performing the following, for example: resolution conversion comprising appropriate magnification modification and supplementation processing on image signals having resolution levels that do not match the number of display elements of the image display portion; conversion of the image renewal frequency; un-interlacing processing; or color matrix conversion. Further, reference numeral 82 is a memory made of image storage area needed for the image processing by the format converter. Reference numeral 82a is a group of control lines of this memory, and reference numeral 82b is a data line group for communicating data between this memory and the format converter. Reference numeral 83 is a crystal oscillator. Based on a clock (OCLK) formed by the crystal oscillator, and in accordance with controls of a microcomputer unit not shown in the diagram, the format converter 81 forms a horizontal synchronization signal (OHD) and a vertical synchronization signal (OVD) for achieving synchronization after the format conversion. Reference numeral 811 is a signal line for the horizontal synchronization signal (OHD), reference numeral 812 is the signal line for the vertical synchronization signal (OVD), and reference numeral 813 is signal line for the clock (OCLK) formed by the crystal oscillator.
Reference numeral 4 is a unit of adjusting image qualities, for receiving the image signal after format conversion, and adjusting brightness, color characteristics, gamma characteristics, and other image qualities in the display portion according to the microcomputer unit not shown in the diagram.
Reference numeral 85 is a PWN converter for converting a sequentially scanned normal image signal into a time division display signal based on the pulse width modulation. Reference numeral 851 is a storage unit for the time divisional drive sequence describing data order after the PWM modulation, and describing a display period. Reference numeral 854 is a PWM drive timing generator for receiving the time divisional drive sequence and generating a drive timing for the PWM converter and the spatial modulation element, which is the image display unit. Reference numeral 851a is a drive sequence data transmission line from the time divisional drive sequence describing unit to the PWM drive timing generator. Reference numeral 855 is a group of control lines for the drive pulse generated by the PWM drive timing generator, for example. Further, reference numeral 856 is an output terminal for outputting a control signal for the drive pulse, for example, that is transmitted to the image display element 2, and other control signals. Further, reference numeral 857 is a data bus for the image data converted by the PWM converter, and reference numeral 858 is an output terminal for outputting the image data for the image display element 2.
The PWM converter control signal and the display element drive pulse are generated at the PWM drive timing generator (reference numeral 854), in accordance with the sequence data in the time divisional drive sequence describing unit. This produces the result that the image inputted to the signal processing unit is converted to an appropriate format and adjusting of the image qualities is performed; and then, is converted into the time divisional drive signal at the PWM converter 85 and the PWM converter and the display element are synchronized and driven.
Reference numeral 86 is a unit of controlling filter motor for forming a signal for performing motor control of the rotating filter drive unit. Reference numeral 861 is an input terminal for the detection signal from detection sensor for detecting the color filter plate rotational phase, and reference numeral 862 is a signal line thereof. Reference numeral 86 receives the synchronization signal (OVD) of the image signal of the output system from reference numeral 812 and the filter phase from reference numeral 861, and performs control such that synchronization of the two may be achieved inside the unit of controlling motor 86, and a motor control signal for correcting slips in synchronization is outputted from a motor control signal output terminal 864 via the line signal 862.
FIG. 11 is an example showing a display data array after the PWM modulation by the PWN modulator 85. In this figure, the horizontal direction indicates time, and the reference numeral 601 is a start pulse of a screen display in the field in red, green and blue.
The period indicated by reference numeral 602 is a period R; the period indicated by reference numeral 603 is a period G; and a period B follows after this period 603.
Reference numeral 604 is the PWM modulated display data for R. Here, for the sake of simplicity, a 6-bit signal is being shown. Reference numeral 606 is a first bit, 607 is a second bit, 608 is a third bit, 609 is a fourth bit, 610 is a fifth bit, and 611 is a sixth bit. The second bit is twice as long as the first bit, and the third bit is twice as long as the second bit, and so on, such that with each next bit the length of the pulse doubles. The signal is modulated to a pulse width corresponding to these bits and the light is reflected at the spatial modulation elements. Thus the image of each color in each field is displayed according to the integral of the period of each color in one field. Similarly, reference numeral 605 is the PWM modulated display data of R. Reference numeral 612 is a first bit, 613 a second bit, 614 a third bit, 615 a fourth bit, 616 a fifth bit, and 617 a sixth bit.
Here, in the spatial modulation element, the period of reference numeral 618 is a time of non-display between the display periods of B and R. Reference numeral 619 is a display period of R, and reference numeral 620 is a time of non-display between the display periods of R and G. Reference numeral 621 shows a display period of G.
Here, consideration is made of a span of a spot of irradiated light on color filter plates 31, 32, and positional relationship of color boundaries of the color filter. In FIG. 12, reference numeral 701 indicates a spot of irradiated light on the color filter plates 31, 32. Reference numeral 702 indicates a position corresponding to the spatial modulation element irradiating on the spot. This, therefore, shows an outline of the spatial modulation element at the spot. Further, reference numeral 703 is the rotational center of the color filter plate. Reference numerals 704 and 705 indicate positions where the boundaries of the color filters of different colors transverse points represented by reference numerals 706 and 707, respectively. As is clear in FIG. 12, the time at which a color filter boundary transverses the point in the spatial modulation element corresponding to reference numeral 706, and the time at which a color filter boundary transverses the point corresponding to reference numeral 707 are different. Therefore, during a period between these two times, light in two colors is irradiated on the screen of the same spatial modulation element.
Returning again to FIG. 11, reference numeral 622 indicates a color period of the color filter at the reference numeral 706 point in FIG. 12, reference numeral 623 is the period R, and reference numeral 624 is the period G. Further, reference numeral 625 indicates a color period of the color filter at the reference numeral 707 point in FIG. 12, reference numeral 626 is the period R, and reference numeral 627 is the period G.
As may be understood from FIG. 11, during the periods indicated by reference numerals 628 and 629, light in two difference colors is irradiated at the same spatial modulation element screen. In the case of the rotating color filter, color mixing occurs while the spot light is passing through the filter boundary. Further, the same sort of problem occurs in the case when a color filter of liquid crystal is used in a switching fashion. In this case, color mixing occurs during the period of time necessary for the liquid crystal to respond to the switching of color filters for each color. Therefore, placing importance on color purity, there are cases when this period is treated as a non-display period and is not used. Further, even though color mixing occurs as shown in the figure, there are cases when this period is used as a display period, just as it is in order to obtain increased brightness. In either case, at the boundary portion of the different color filters there is an unusable portion having at least the range indicated by 620.
However, in display devices having such a sequential, color-switching format, the construction and characteristics of the color filter plate are a significant factor in determining the performance of the display device. For example, the brightness and color characteristics of an image are such that when efforts are made to improve the one, the other suffers. Thus, there is a trade-off relationship between the two (i.e., a relationship such that in order to improve the performance of one it is necessary to sacrifice the performance of the other. Hereinafter, this term is used for the same meaning). Hereinafter, explanation is made of this point.
That is, the construction and the characteristics for the color filter are selected from a host of aspects that are in a trade off relationship, such as brightness, color characteristics and performance speed. Therefore, in conventional image display devices it was difficult to optimize for diverse image sources so as to further improve image qualities.
For example, with respect to the relative portion of the circle occupied by a color and the transmittancy characteristics of that color, there is a trade off relationship that obtains between the brightness and the color characteristics. Further, with respect to the number of divided filters, there is a trade off relationship obtaining between improving the image qualities by accelerating the switching of the illumination light colors, and brightness and color characteristics.
With respect to the portion of the circle that each color occupies, an example may be given for performing processing for emphasizing white brightness. In addition to the RGB color segments, a white (W) area may be set, through which all wavelength components transmit. In white-brightness-emphasization processing, which is for emphasizing an apparent brightness, the peak brightness of white increases, but the filter surface area of each of the RGB color components decreases. Thus, the range is diminished during which the brightness of each pure color is reproduced, and the level of color purity declines.
An explanation is now made of the white-brightness-emphasization processing. In the color field sequential display format for sequentially displaying the signals RGB of each of the colors, the signals of G and B are discarded when R is displayed; R and B are discarded when G is displayed; and G and R are discarded when B is displayed. Therefore, the light usage ratio is basically at the level of ⅓, so there is insufficient brightness. In order to augment this, there is a method known in U.S. Pat. No. 5,233,385, for example, for artificially emphasizing white brightness in a white signal exceeding a predetermined signal level, by using a white-brightness-emphasization signal to display in a white display area. FIG. 13 is an example of a construction of a color filter of a display device having a function for such white-brightness-emphasization. 31R is a color filter (color area) designed to transmit red color components, 31G is the same for green color components, and 31B is the same for blue color components. Here, 31W is an area used for emphasized display of white. Reference numeral 31a is rotational center portion attached to a rotation axis.
FIG. 14 is an explanatory diagram of display brightness levels displayed by a display device. The vertical axis indicates the display brightness of the display device.
The solid lines 901 and 904 indicate display brightness levels corresponding to signal levels changing, in steps, from 0% to 100%. Further, the broken line 906 indicates brightness levels from 100 to 200%, realized by means of the white-brightness-emphasization processing. 902 and 903 are points on 901; 907 and 909 are points on 904; and 908 and 910 are points on 906. Further, the bold broken line 905 is the display brightness of a white display portion produced by the white-brightness-emphasization signal. In the normal situation, in which white-brightness-emphasization processing is not performed, display is possible to the level of 100% indicated by 901. Normally, signals corresponding to this signal are provided for each of the colors RGB, and are displayed with a brightness in the range of 0 to 100%. Therefore, a brightness range from 902 to 903 becomes the display range of the display device. In the case when white-brightness-emphasization processing is performed, the display device extracts the white signal common to R, G and B from signals exceeding a predetermined signal level, and performs a display corresponding to the white-brightness-emphasization signal 905 in the white area. Further, the differential signals remaining after the white signal is extracted from each of the color signals are displayed in the display areas of each of the colors. 904 corresponds to the display by this differential signal. As a result, the display brightness level that is finally realized is a brightness level 906 being a composite of 904 and 905. As a result, display brightness levels 907 and 909 are converted to 908 and 910 respectively and displayed.
As described above, white-brightness-emphasization processing is an effective method for emphasizing the display peak of white. However, the display range of each of the single colors R, G and B is only 0 to 100%, while white alone is 0 to 200%. Thus, the color purity of the display device diminishes. The issue of whether to perform the white-brightness-emphasized display or not, and the issue of the proportions at which each of the color areas, including white, should be set, involve a trade off relationship between brightness and color purity. If one of these is improved, it is necessary to sacrifice characteristics of the other.
For the transmittancy characteristics of each color, it is possible to set a slightly wide range for the transmission wavelengths of the filter for each color to produce increased brightness. However, it has been pointed out that this produces deteriorated color characteristics. In FIG. 15, an example is shown of transmittancy characteristics of filters of each color R, G and B. The horizontal axis is the light wavelength in terms of nm. The vertical axis is the light transmittance in terms of percentage. 1001 is the transmittancy characteristic of R; 1002 is the transmittancy characteristic of G; and 1003 is the transmittancy characteristic of B. According to the example of FIG. 15, the transmittancy wavelength area of R and the transmittancy wavelength range of G are overlapping. Therefore, it may be said that color purity is suffering, but the brightness is increasing. When the overlap of the wavelength range is decreased, brightness decreases while the color purity is improved. In this way, brightness and color purity stand in a trade off relationship such that if one is improved then the characteristics of the other must be sacrificed.
In connection with the issue of the number of filter divisions, an example may be given of an approach for solving the color-split phenomenon. With the sequential, color-switching format, there is known the following unique problem: each of the images R, G and B should be composited against a quickly moving line of vision all at the same position visually. However, the positions of images R, G and B appear to slip from that position, and a phenomenon of color splitting occurs (color breakdown phenomenon). In order to suppress this phenomenon, there exists a method of increasing the number of divisions and rotations in each filter (i.e., in each color area) in the color filter plate, and increasing the renewal wavelength of the screen. However, increasing the number of divisions of the filter increases the unusable boundary region of the filter as explained in FIG. 11, which leads to reduced brightness. Further, when the usable range of the boundary portion is increased for the sake of better brightness, the level of color purity drops. In this way, acceleration for purposes of resolving the color split phenomenon (or color breakdown phenomenon) stands in a trade off relationship with both brightness and color purity. If one of these is improved, the characteristics of the other suffer.
Note that, Japanese Patent Application Laid-open No. 5-83722 and Japanese Patent Application Laid-open No. 2000-347287 disclose other background technologies.